1. Field of the Invention
The invention relates to a device for counting fine particles that are in the atmosphere or contain certain gases. The invention further relates to a device for counting, in particular, fine particles that are in a clean room and contain a starting gas for the production of semiconductors.
2. Description of Related Art
It is generally known to utilize a measuring device that uses condensation nuclei as a device for counting fine particles that are suspended in the atmosphere or are contained in certain gases in a semiconductor production process.
In such a measuring device, gas that contains fine particles is introduced into a chamber for saturated vapor, so that it contains liquefied vapor. By cooling the gas containing liquefied vapor in a condensation part, and condensing the vapor on the fine particles as nuclei, an apparently enlarged fine grain size is obtained, due to which a subsequent detection process of the particles in a particle measuring part is made possible in a simple way and with increased accuracy.
In a conventional measuring device, alcohol is usually used as the liquid for condensation. Here, a condensation part is cooled to a temperature less than or equal to a room temperature and alcohol vapor is condensed. It is true that this process is for gases with a low flow rate, but for gases with a high flow rate it has a drawback that it needs a large cooling mechanism and thus is difficult to use in practice.
FIG. 1 illustrates a prior art measuring device using glycerine as the condensation liquid. In the illustration, a reference symbol 1 designates a chamber for saturated vapor, a reference symbol 2 designates a condensation part and a reference symbol 3 designates a particle measuring part using a light-scattering system.
Inside chamber 1 for saturated vapor, glycerine 4 is encapsulated as the liquid for condensation, and it is heated by a heating means 5 placed under it. Condensation part 2 connected to this chamber 1 for saturated vapor consists of narrow pipes 6 and is exposed, without a special cooling means, to a room temperature.
Attached particle measuring part 3, to which condensation part 2 is connected by a connection point 25, has the same arrangement as a usual air counter and is provided, on a side with a light source, with an optical system that consists of a lamp or laser 7, a lens 24 for light collection, a slit 9 and a projection lens 10 and, on a light collection side, with an optical system that is placed opposite the above-described optical system, and consists of a convex lens 11 and a device 12 for fight collection.
Gas containing fine particles is introduced out of an inlet 1 a into chamber 1 for saturated vapor, which is filled with a saturated vapor of glycerine heated by heating means 5. Next, the introduced gas is mixed with a glycerine vapor in this chamber 1 for saturated vapor.
Next, the mixed gas is fed to condensation part 2. In narrow pipes 6 of condensation pan 2, whose temperature is about at a room temperature, the glycerine vapor grows by condensation on the fine particles contained in the gas as nuclei.
Fine particles grown like this are optically measured in connected particle measuring pan 3. Particle measuring part 3 has, as described above, an open design in which no partition is placed between a gas flow line and the optical systems, such as lenses and the like. Here, light from laser 7 is focused using the optical systems and irradiates a detection area. Scattered light produced when the grown particles in the mixed gas pass through the laser light is detected by convex lens 11 in the device for light collection 12. In doing so, a penetration of directly projected light from a lamp into the optical system on the light-collecting side is prevented by a beam stopper 17.
Such a prior art device is known from the references cited below:
A counting of particles using condensation nuclei is described in Japanese laid-open specification SHO 57-42839 entitled "Process for counting superfine particles and device for performing the process". A counting of particles using the light-scattering system is described in Japanese laid-open specification SHO 64-53132 entitled "Particle detector," in Japanese laid-open specification SHO 64-53131 entitled "Particle detector" and in Japanese laid-open specification SHO 55-39772 entitled "Particle measuring pan."
Because of the fact that, in contrast to using alcohol vapor at a normal temperature, condensable glycerine is used, the above-described prior art requires no special cooling means in the condensation part and is suitable for detecting particles in gas with a high flow rate.
The temperature of the condensation part is exposed to room temperature and is not regulated. But, it rises slightly above room temperature as a result of the high temperature of the mixed gas, specifically by about 5.degree. to 15.degree. C. above a room temperature of about 25.degree. C. This means that the attached particle measuring part, a connection pipe or nozzle placed between the particle measuring part and the condensation part and whose temperature is about at a room temperature, has a lower temperature than the temperature of the condensation part.
As a result of this, the glycerine vapor condenses on the particle measuring part, the connection pipe or nozzle, and adheres to the optical systems placed in the area. Especially in the gas flow line, which consists of the particle measuring part, the connection pipe and the nozzle, as a result of its design in which there is no partition to the optical systems that focus the laser light, the above-described problem appears extraordinarily frequently. After a long period, such a phenomenon has a negative influence on measuring accuracy, such as degradation of sensitivity, frequent miscounting and the like.
The above-described drawback, i.e., condensation of the vapor on the measuring part, hardly occurs when using alcohol as the condensation liquid, because in doing so the temperature of the condensation part is at a low temperature of less than or equal to room temperature and the measuring part is at a room temperature and thus has a higher temperature than the condensation part. Further, as a result here of a rather high vapor pressure of alcohol at a room temperature, in case the condensed liquid of alcohol adheres to the measuring part, the condensed liquid of the alcohol can be brought back into the previous state and reused by letting it stand for a certain time or by letting gas flow over it.
In contrast, when using glycerine, condensed liquid of the glycerine that once adhered to the measuring part hardly evaporates at a room temperature, since its vapor pressure at room temperature is extremely low and thus is difficult to bring back into the previous state without performing a process such as separation or the like.